In the case of volume rendering, it is also the interior volume, i.e. not only a surface of an inhomogeneous object, that is visualized, and so it is also possible to reproduce transparency effects and/or internal structures in a realistic manner. Here, the three-dimensional object is represented by volume data in three-dimensional resolution.
A known method for the volume rendering is the so-called ray casting, in which a course of imaginary rays, referred to as rays below, is simulated, said rays emanating from the eye of an imaginary observer or from an imaginary detector and extending through the object to be visualized. Illumination values for points within the object are ascertained along the rays. Finally, a visualized two-dimensional image is assembled from the illumination values ascertained for the rays.
A realistic visualization requires effects of the global illumination, such as e.g. surrounding coverage, cast shadows, translucency, so-called color bleeding, surface shading, complex camera effects and/or illumination by arbitrary surrounding light conditions, to be taken into account as comprehensively as possible. Particularly in the case of volume rendering, such illumination effects substantially contribute to the depth and form perception, and hence to an improved image understanding.
Synthetic light sources are often used for illumination purposes in order to calculate realistic shadow representations. Although such synthetic light sources often offer a good shadow representation, they generally impart a synthetic, unnatural look to the synthesized image, while other illumination methods with naturally looking images are only able to synthesize unsharp shadows.
The article “Exposure Render: An Interactive Photo-Realistic Volume Rendering Framework” by Thomas Kroes et al, PLoS ONE, volume 7, issue 7, July 2012, has disclosed a volume rendering method which uses a Monte Carlo simulation for tracking rays. However, further rays need to be tracked within the object volume in addition to a respective ray in order to calculate realistic shadowing effects, causing a significant computational outlay. Moreover, so-called importance sampling is required, which influences the statistical properties of the Monte Carlo method.